The invention relates to a casting unit for use in a diecasting machine, the casting unit comprising a casting chamber body and a casting piston. The casting chamber body has a casting chamber which can be filled with casting material and has a casting material inlet and a casting material outlet. The casting piston can be moved forward in a longitudinal direction of the casting piston in the casting chamber, in order to discharge casting material from the casting chamber under pressure via the casting material outlet, and can be moved back, whereby casting material can be fed into the casting chamber via the casting material inlet.
Such casting units typically serve for conveying a molten metal at high speed and high pressure out of the casting chamber into a die cavity by the action of the casting piston in the corresponding diecasting machines, for example of the hot-chamber or cold-chamber type. In the die cavity, the desired metal casting then forms by the molten metal solidifying. Depending on the casting material, for example alloys of zinc, Al or magnesium, and the casting to be produced, the casting unit has to withstand relatively high temperatures and pressures of the molten metal, for example over 600° C. and 1000 bar, which is known to require special structural design measures.
In the case of conventional casting units, the casting piston is typically formed as a spool, which can be moved axially forward and back in a hollow-cylindrical casting chamber body, its outer cross section corresponding to the inner cross section of the casting chamber body. In other words, this spool forms an axially movable end wall of the casting chamber that variably delimits the casting chamber volume, this conventional type of casting piston sealing the casting chamber volume off at this end face by its outer cross section corresponding to the inner cross section of the casting chamber body, possibly assisted by assigned sealing devices that are arranged for example on the outer circumference of the piston. The force transmission to the casting piston takes place via a piston shaft that is provided on the extreme end of the casting piston facing away from the casting chamber and has a cross section smaller than that of the casting piston. The casting piston shaft may for example be led through an associated through-passage in the casting chamber body out of the latter, this through-passage then having a cross section which corresponds to that of the piston shaft and is smaller than the outer cross section of the casting piston and the inner cross section of the cylindrical casting chamber body.
Various conventional casting units are disclosed for example in the laid-open patent applications DE 10 2005 009 669 A1, DE 195 44 716 A1 and DE 43 16 927 A1 and also in the patent specification EP 1 483 074 B1.
Casting units with said type of spool present some specific technological challenges. One problematic aspect is the effect of so-called skin solidifying. The comparatively cooler cylinder wall of the casting chamber body may cause molten material to harden on its inner wall and disturb or hinder the movement of the casting piston moving in a sealing manner along it with two-dimensional area contact. Moreover, with the casting piston moved back, in the casting chamber there is not only casting material but usually also air, which has to be driven out again during the die-filling operation, i.e. when moving the casting piston forward, or may lead to problems of the molten material oxidizing.
It is an object of the invention to provide a casting unit for a diecasting machine with which the aforementioned difficulties of conventional casting units of the spool type can be eliminated or at least reduced.
The invention achieves this object by providing a casting unit comprising a casting chamber body and a casting piston, where the casting chamber body includes a casting chamber which can be filled with cating material and has a casting material inlet and a cating material outlet. The casting piston is capable of being moved forward in a longitudinal direction of the casting piston in the casting chamber, in order to discharge casting material from the casting chamber under pressure via the casting material outlet, and moved backward, in order to feed casting material into the casting chamber via the casting material inlet. The casting piston extends through a through-passage of the casting chamber body from outside into the casting chamber, an area of free space of the casting chamber being formed between an outer lateral surface of the casting piston moved forward into the casting chamber and an inner wall surface of the casting chamber body lying opposite said outer lateral surface transversely in relation to the longitudinal direction of the casting piston, by an outer cross section of the casting piston being appropriately smaller than an inner cross section of the casting chamber body.
In other words, in the case of the casting unit according to the invention, the casting piston is of a displacement type, which reduces the casting chamber volume appropriately by moving forward into the casting chamber, without coming to lie with its outer cross section against the inner cross section of the casting chamber body in a sealing manner over its entire surface area like a conventional spool. Leaving the area of free space does away with any problems of friction between the outer cross section of the casting piston and the inner cross section of the casting chamber body lying opposite transversely in relation to the longitudinal direction of the casting piston, for example as a result of the mentioned effect of the skin solidifying. Thus, any problem of friction caused by two-dimensional, surface-area frictional contact can be limited locally to the region of the through-passage. This can be controlled much more easily than the conventional problem of friction between the outer cross-sectional area of the casting piston and the inner cross-sectional area of the casting chamber body along the entire length of displacement in the case of the conventional type of spool. If need be, an only one-dimensional, linear or zero-dimensional, punctiform guiding contact may be retained between the casting piston and the casting chamber delimiting wall. Furthermore, this design of the casting unit according to the invention is a comparatively easy way of offering the possibility of keeping the casting chamber completely filled with casting material at all times, without ambient air inevitably getting into the casting chamber.
In a development of the invention, the casting material inlet opens out into the area of free space and/or into the casting material outlet of the casting chamber. This advantageously has the consequence that, even with the casting piston moved forward to the maximum, the casting chamber inlet is not blocked by the latter. Thus, even at the beginning of the movement back of the casting piston from its position of having been moved forward to the maximum, casting material can already be fed into the casting chamber via the casting inlet. By contrast with this, in the case of conventional casting units of the spool type, the casting inlet is usually blocked by the casting piston that has been moved forward and is only released by it when the casting piston has moved back a certain amount from its position of having been moved forward to the maximum. The present casting unit consequently makes a comparatively uniform, homogeneous feeding of casting material into the casting chamber possible, and consequently also the avoidance of undesired turbulences and undesired sucking in of ambient air via the casting material outlet when the casting piston is moved back. The casting chamber can consequently be readily kept completely filled with casting material at all times.
In a further refinement, the casting material inlet and/or a casting material feed line assigned to it is provided with a shut-off element, which prevents casting material from leaving the casting chamber via the casting material inlet. Depending on requirements and the application, this may be an actively or passively acting shut-off element of a conventional type known per se, for example an appropriate check valve.
In a development of the invention, the casting chamber body has a hollow cylinder, and the through-passage is provided at an extreme end of the same. The casting piston may then for example extend with the longitudinal axis of the piston that is parallel to the longitudinal axis of the hollow cylinder axially via the through-passage into the casting chamber. In a further refinement, the casting material outlet and/or the casting material inlet is provided at the extreme end of the hollow cylinder that is opposite from the through-passage or on the cylinder lateral surface of the hollow cylinder. These positioning measures may contribute to favorable flow characteristics for the casting material that is to be introduced into the casting chamber and casting material that can be discharged from it under pressure into a die cavity.
In a development of the invention, a guiding sleeve is provided for the casting piston, said sleeve extending outward from an outer side of the through-passage that is facing away from the casting chamber and/or extending from an inner side of the through-passage that is facing the casting chamber into the casting chamber. With this guiding sleeve, the casting piston may be additionally supported and guided during its movement forward and back.
In a development of the invention, a sealing element for sealing off the casting piston passage is provided. In one possible way of realizing this, the sealing element is arranged on an inner side of the through-passage or of the guiding sleeve that is facing the casting chamber. Arranging it on the inner side has the advantage that, should a solidifying effect occur in this region, solidified molten material can be forced back into the casting chamber without any problem when the casting piston moves forward, without causing disturbing frictional effects between the casting piston and the inner wall of the casting chamber body. Also when the casting piston moves back, molten material that has possibly solidified on the inner side of the through-passage or the guiding sleeve in the region of the sealing element does not cause any problems, if only because, by contrast with the movement forward of the casting piston, this movement back can take place with virtually no pressure. This is so because, during the movement back of the casting piston, the casting material in the casting chamber is not under the high pressure such as that prevailing during the die-filling phase when the casting piston is moving forward, but is pressureless or at most under a much lower feed pressure, which may optionally be used for replenishing the casting chamber with casting material.
In a development of the invention, a casting piston temperature control device for actively controlling the temperature of the casting piston, at least in certain regions, is provided. It is thereby possible, according to requirements and the application, to have an active influence on the temperature of the casting piston, the part of which that is respectively in the casting chamber being subjected to the effects of the temperature of the hot casting material that is present there. In a refinement of this measure, the casting piston temperature control device is designed for allowing the temperature of the casting piston to be actively controlled according to a predeterminable temperature profile along at least part of its length. For example, this may involve suitably compensating partly or completely for a temperature influence of the hot casting material in the casting chamber on the casting piston that leads to a temperature gradient along the casting piston.
In a development of the invention, a casting chamber temperature control device for actively controlling the temperature of the casting chamber is provided. This may be used for example for preventing effects of molten material solidifying in the casting chamber or for achieving a relatively homogeneous temperature distribution of the casting material in the casting chamber.
In a development of the invention, the casting unit has a relieving annular groove and a relieving channel, the relieving annular groove being located on an inner wall of the through-passage or the guiding sleeve that is facing the casting piston and the relieving channel being led from the relieving annular groove to the outer side of the casting chamber body. If some molten material or other fluid gets between the casting piston and the through-passage or guiding sleeve, for example owing to wear, it can be led away to the outside in a controlled manner via the relieving annular groove and the relieving channel.